The present invention relates to a process and to an apparatus for obtaining an excimer usable as a light source, emitting in the ultraviolet or visible, in a pulsed laser with a high repetition rate or in a continuous laser.
The invention is applicable to lasers in general and more specifically to those with a high power and efficiency level, more specifically usable in the fields of photochemistry,telecommunications and controlled fushion.
The development of gas lasers of excimers in the field of visible and ultraviolet wavelengths has been the object of considerable interest over the last ten years. This is partly due to the great diversity of wavelengths and to the narrowness of the spectral range of laser emissions (a few nm).
An excimer is an excited chemical species in the gaseous state which, on deexciting or deenergizing, can be chemically dissociated during a light emission, the latter corresponding to an electronic transition between the excited state of the excimer and its very slightly bound normal state (approximately 100 to 300 cm.sup.-1).
Due to the very fast chemical dissociation of the molecule in the normal state, the population inversion is easily realisable and the effective stimulated emission section has a high value (approximately 10.sup.-16 cm.sup.2).
For a general description of the principles and characteristics of excimer lasers, reference can e.g. be made to the article by J.J. Ewing entitled "Excimer Lasers", published in Laser Handbook, edited by M.L. Stitch, North Holland Publishing Company 1979, pp. 135 to 197.
In excimer lasers, the active medium is constituted by a gaseous mixture R+XY, in which R generally represents a rare gas such as xenon, argon or krypton and XY generally designates a halogenated, diatomic gaseous molecule such as CL.sub.2, F.sub.2, HCL or HF. This active medium is immersed in a buffer gas constituted by a rare gas identical or different to R.
One of the gaseous reagents R or XY is excited or ionized either by optical pumping, or by electron bombardment, or by electric discharge and collision of the excited reagent with the other reactant produces a rare gas halide RY.sup.* in an excited state, which is the precursor of the laser emission.
The product RY.sup.* is mainly formed in two excited electron states called B and C, but the laser emission corresponds to the electron transition between level B and the normal level X. This laser emission mainly takes place as from the normal vibronic level of state B (v=0), populated by collisional relaxation of the excited states of B (v.noteq.0) with the aid of the buffer gas.
The reaction mechanism can be diagrammatically shown as follows: ##STR1## M being the buffer gas in which the mixture R+XY is immersed.
FIG. 1 shows the potential curves of XeCL corresponding to R=Xe and XY=CL.sub.2. These curves, which represent the above reaction mechanism, have in particular been published in the article by P.J. Hay et al entitled "The covalent and sonic states of the xenon halides", published in J. Chem. Phys. 69, 1978, pp. 2209. These curves give the bond energy E, expressed in eV, as a functional of the distance of the bond r of XeCL, expressed in nm.
FIG. 1 gives the potential curve of the system XeCL. The excited state (curve 1) corresponds to the state Xe.sup.+ (.sup.2 P.sub.3/2) and CL.sup.-. It can be gathered from curve 1 that the bond energy of the excimer XeCL.sup.* (B) is below those of Xe.sup.+ and CL.sup.- which are not bound.
Curve 2 gives the bond energy of the xenon and chlorine molecules in the non-excited state.
The light emission symbolized by the arrow F is due to the deexcitation of the excimer XeCL.sup.* (B,v=0), which is accompanied by the dissociation of the latter into xenon and chlorine atoms in the non-excited state (curve 2). The wavelength of the light emitted is 308 nm.
One of the disadvantages of the presently known excimer lasers is their poor efficiency of approximately 1%, which is defined by the ratio of the laser energy extracted to the energy absorbed by the active medium during electron bombardment.
Another major disadvantage of these excimer lasers is the limited regeneration of the starting gas making it difficult to operate at high frequency and impossible to operate under continuous conditions. In particular, the recycling of the gas is generally performed by ventilators limiting the number of laser pulses per unit of time, the repetition rate of said lasers being approximately 100 Hz.
In order to increase the collision probability between the excited reagent (e.g. R*) and the other reactant (XY) and consequently obtain high laser powers, it is necessary for the reagents to have a high pressure (approximately 10.sup.4 Pa). In the same way, the buffer gas must have a high pressure (approximately 10.sup.5 Pa) in order to vibrationally relax the excimers formed.
Unfortunately it is difficult to control the production of an electric discharge for exciting one of the two reagents in a high pressure medium. Moreover, the voltages conventionally used for bringing about this discharge in an effective manner are high and range between 25 and 30 kV.
In order to obtain a higher gas flow and a constant replenishment of the active medium, a supersonic flow laser has been developed. The latter is more particularly described in the article by B. Fontaine et al entitled "Nouveaux lasers supersoniques a tres basse tempeerature et haute densite", published in the journal of Cethedec 19, special number 1982.2, pp. 55-80 and in the article "Les lasers a excimeres avec ecoulement gazeux supersonique" by B. Fontaine et al, published in Entropie, no. 89-90, 1979, pp. 118-125.
In this device, the supersonic flow is obtained with a Laval nozzle and the electric discharge used for exciting one of the reagents R or XY is stabilized or assisted by an electron beam or X-ray. The reaction diagram is identical to that given hereinbefore.
Furthermore, the density of the gas (or pressure) in the supersonic flow must still be maintained high in order to assist collisions between the excited reagent and the other reagent and the relaxation of the species RY.sup.* (B,C). Unfortunately the cooling of the gas due to the supersonic expansion contributes to the formation of aggregates between the reagents, thus limiting the concentration of the reagents available for forming the excimer RY.sup.*.
In particular, in the case of a reactive mixture Xe+HCL, the formation of aggregates of (HCL).sub.n is high, n being a positive integer exceeding 2. The disadvantage of these aggregates is more particularly described in Appl. Phys. Lett. 36(3) of 1.2.1980 by B. Fontaine and entitled "High specific power long-pulse supersonic flow XeCL laser at 308 nm", pp. 185-187.
The inventors have found that the formation of these aggregates of (HCL).sub.n was due to the high polarity of the HCL molecule.
Other aggregates or homogenous complexes of type R.sub.2.sup.+ (e.g. Xe.sub.2.sup.+) are also produced via ion-molecule reactions, whose effective sections are very high under the temperature and pressure conditions of supersonic flow. These two effects limit the laser efficiency to approximately 2.5%.
In the aforementioned supersonic flow laser, the Mach number defining the gas flow rate varies from 1 to 3. This Mach number is relatively low so as to permit collisions between the reagents, necessary for obtaining the excimer. Thus, the number of collisions per unit of time p is a decreasing function of the Mach number: p.perspectiveto.M.sup.-4. Moreover, the use of an electron gun or an X-ray generator make the known supersonic flow lasers complex and therefore costly.
Hitherto every effort has been made to produce collisions between the reagents, in order to obtain a large number of excimers and therefore the maximum light power. However, contrary to the objective sought in the articles by B. Fontaine et al and contrary to all logic, the inventors have removed the collisions and sought to increase the number of Van der Waals complexes of the R-XY type in the gaseous mixture by increasing the Mach number of the supersonic flow and by using reaction mixtures formed by weakly polarized or non-polar molecules. Moreover, they have sought to directly excite these Van der Waals complexes. They have been able to obtain the emission of the excimer wtih a narrow spectral band (approximately 10 nm).
It should be noted that the formation of Van der Waals complexes (Xe-Br.sub.2, Xe-CL.sub.2) the optical excitation of these complexes forms the subject matter of a publication entitled "Formation of the XeBr.sup.* excimer by double optical excitation of the Xe-Br.sub.2 Van der Waals complex" by M. Boivineau et al, published in J. Chem. Phys. 84(8) of 15.4.1986, pp. 4712-4713, but excitation of the complexes was then performed by a complex optical pumping device.